Unsaturated hydrocarbons, e.g. olefins, may be derived by catalytic dehydrogenation of an alkane to create a product more useful and more valuable than the reactant. Olefins are feedstocks for many processes that yield increasingly valuable products through polymerization. Catalytic dehydrogenation of alkanes is a selective way to produce alkenes and was commercialized in the 1930's.
One such catalytic dehydrogenation process is the CATOFIN® process which produces alkenes, e.g. isobutylene, propylene and amylenes, from alkanes, e.g. isobutane, propane and isopentanes respectively. The CATOFIN® process is the most widespread catalytic dehydrogenation process employed in the production of propene, the precursor material to polypropylene. Another dehydrogenation process is the CATADIENE® process which results in the catalytic dehydrogenation of C4 and C5 hydrocarbons to produce diolefins, i.e. dienes, of the same carbon number. The CATOFIN® and CATADIENE® processes utilize a supported chromium catalyst in a series of adiabatic fixed bed reactors in cyclic operation with quickly alternating dehydrogenation and regeneration periods. Other catalytic dehydrogenation processes utilizing supported chromium catalysts are known in the art. Such processes typically employ frequent high temperature regeneration cycles.
Catalysts that are used for the dehydrogenation of light hydrocarbons typically comprise chromium oxide supported on the surface of an aluminum oxide carrier, i.e. chromium-alumina catalysts. Processes utilizing supported chromium catalysts such as chromia-alumina catalysts (Cr2O3/Al2O3) are well known and have been described in technical literature as well as in numerous patents.
Dehydrogenation catalysts become spent after prolonged use through several alternating dehydrogenation and regeneration cycles. Spent catalyst must be removed from the reactor and replaced with fresh catalyst. It is preferred that at least some and preferably a majority of the chromium in the spent catalyst be at a reduced oxidation state when the spent catalyst is removed from the reactor. The applicant has now surprisingly found a method for shutting down a dehydrogenation reactor which method results in at least some and preferably a majority of the chromium in the spent catalyst being at a reduced oxidation state.